The present invention relates to a device for sealing materials such as plastic film and, more particularly, to an improved heat-sealing device having a heat-source encapsulated within a thermal conductor.
Various types of machines exist for forming containers from plastic films. In such machines, one or more heat-sealing devices are included for sealing together the plastic films in such a manner as to create and/or seal-closed the containers.
In the field of packaging, for example, many types of machines form inflated packaging cushions by inflating a flexible container, e.g. a bag, with air, and then sealing closed the inflated container. The inflatable containers may be pre-formed and arranged in series in a flexible web, with only a longitudinal closure seal being formed at the opening of the containers by the heat-sealing device, wherein “longitudinal” refers to the direction in which the web moves as it is conveyed through the machine. Alternatively, the containers may be formed from a pair of juxtaposed film plies, wherein one heat-seal device forms a longitudinal seal between juxtaposed edge regions of the films to form a closed longitudinal edge, while leaving the opposing longitudinal edge open; another heat-seal device creates transverse seals between the two film plies to form the containers, with the open longitudinal edge providing openings in the containers for inflation; and a third heat-seal device forms a longitudinal seal at the open longitudinal edge to seal-closed the openings after the containers have been inflated. Alternatively, a single film ply may be used, which is ‘center-folded’ in the longitudinal direction such that the fold forms the closed longitudinal edge; in this case, only one longitudinal heat-seal device is required. Examples of such machines may be found, for example, in U.S. Pat. Nos. 6,598,373, 7,220,476, and U.S. Pat. No. 7,225,599.
Another method for producing packaging cushions is known as ‘foam-in-place’ packaging, wherein a machine produces flexible containers, e.g., bags, from flexible, plastic film, and dispenses a foamable composition into the containers as they are being formed. As the composition expands into a foam within the container, the container is sealed shut and typically dropped into a carton, e.g., a box, which holds the object to be cushioned. The rising foam expands into the available space within the carton, but does so inside the container. Because the bags are formed of flexible plastic, they form individual custom foam cushions around the packaged objects. As part of the container-forming mechanism, a heat-seal device is generally provided for forming a longitudinal heat-seal. Exemplary types of such packaging apparatus are described, for example, in U.S. Pat. Nos. 4,800,708, 4,854,109, 5,027,583, 5,376,219, 6,003,288, 6,472,638, 6,675,557, and U.S. Pat. No. 7,607,911, and in U.S. Pub. No. 2007-0252297-A1.
While the foregoing machines for making air-filled and foam-filled packaging cushions have been widely used and commercially successful, improvement is continually sought. One particular aspect wherein improvement is desired concerns the manner in which the film plies are sealed together, especially in the longitudinal direction, i.e., the direction in which the film plies move as they are conveyed through the machine.
The inventors hereof have determined that an important factor in making good heat seals is consistency in the temperature at which heat is applied to the films during the formation of the seal. The selection of the correct temperature to be applied during heat-sealing is commonly carried out by operators of cushion-making machines through routine experimentation, e.g., by trial and error. If the temperature is too high, the heat-seal device may melt through the films without sealing them together; if the temperature is too low, no seal or an incomplete/weak seal may be formed. The correct temperature to be selected will vary from application to application, based on a number of operational factors, including the composition and thickness of the film plies to be sealed, the pressure at which the film plies and the heating device are urged together, the speed at which the film is conveyed, etc. Mathematical algorithms may also be used to select to optimum temperature, e.g., based on operator input and/or sensor input of the foregoing factors.
In addition to the selection of the proper heat-sealing temperature, a factor that is equally important to the formation of good, consistent heat seals is the ability of the heat-seal device to maintain the selected temperature during the formation of the heat seals. A number of factors can influence the temperature of the heat-seal device, including the speed at which the film is conveyed through the machine. In many packaging-cushion machines, the film is driven at varying speeds through the machine. As the film is driven faster, it has more ability to remove heat from the heat-seal device, necessitating higher wattage (electrical power) to maintain the proper temperature. Conversely, as the film drives more slowly, it does not use the heat as fast, requiring less power to make the seal. Other factors involved in determining the power necessary to make a sufficient seal include ambient temperature, latent heat build-up in the sealing components, the thickness of the film material, and the temperature of the film itself, e.g., a new roll of film may be taken from a cool storage room and installed on the machine, where it will slowly raise to ambient temperature.
While conventional packaging-cushion machines typically have means for controlling the temperature of the heat-seal device to achieve consistency, improvement is sought in order to obtain a higher degree'of precision, i.e., a lower degree of temperature variation from a selected temperature.
Another aspect of conventional heat-seal devices for which improvement is sought concerns the structure of such devices. Conventional heat-seal devices employ, as a heat-source, an electrically-resistive heating element, which generates heat upon the passage of electricity therethrough. Such heating elements are typically fully exposed, and brought into direct contact with the film to be sealed, which can result in melt-throughs of the film plies. When the heat-seal device melts through the film plies, an outer strip from one or both film plies very often separates from the rest of the film and wraps around the heat-seal device. This problem, which is known as ‘ribbon cutting,’ results in the necessity of shutting down the cushion-making machine and extricating the film strip from the heat-seal device. Typically, the strip is tightly wound around the device and/or partially melted such that removal of the strip is a difficult and time-consuming process. Another disadvantage of ‘open-air’ heating elements is that such configuration limits the service life of the heating element due to frictional contact with the film and oxidation due to exposure to the air while heated.
Therefore, the need exists for an improved heat-seal device that is suitable for forming heat-seals in packaging-cushion machines, and which avoids the foregoing disadvantages.